Cyproheptadine is currently clinically used as an appetite stimulant for children with failure to thrive without underlying organic disease. Otherwise, no randomised control trial demonstrates the efficacy of Cyproheptadine on those patients. This is precisely what the investigators intend to demonstrate on this randomised placebo control cross-over trial. Our hypothesis is that Cyproheptadine is more efficient than placebo to improve weight gain and feeding behaviour on 2 to 4 years old children with failure to thrive.